Sounds like the MC, had this problem on my 74 Nova only did it once and a while. What I found was if you would “very gently” push on the pedal it would go to the floor…try that…if you used normal pressure it was fine. What happens is if you gently push on the brake and it goes tom the floor, it means the brake fluid is getting past the seals. When braking normally the extra pressure expands the seals enough to provide normal braking.
Replace the MC, and flush the old brake fluid and air out of the system.